Could you test the game in a new profile that has no history of ever having had Flash? I'm curious whether the game is no longer compatible with Firefox, or whether there might be a setting or add-on we need to root out in your regular profile to make it work. Here's how to do the test:

New Profile Test

This takes about 3 minutes, plus the time to test the game.

Inside Firefox, type or paste about:profiles in the address bar and press Enter/Return to load it.

Take a quick glance at the page and make a mental note of which Profile has this notation: This is the profile in use and it cannot be deleted. That is your current default profile.

Click the "Create a New Profile" button, then click Next. Assign a name like Test2024, ignore the option to relocate the profile folder, and click the Finish button.

Firefox will switch your default profile to the new one, so click the Set as Default Profile button for your regular one to avoid an unwanted surprise at your next startup.

Scroll down to Test2024 and click its Launch profile in new browser button.

Firefox should open a new window that looks like a brand new, uncustomized installation. (Your existing Firefox window(s) should not be affected.) Please ignore any tabs enticing you to connect to a Sync account or to activate extensions found on your system to get a clean test.

If you go ahead and sign in to Facebook, does the game work any better?

When you are done with the experiment, you can close the extra window without affecting your regular Firefox profile. (Test2024 will remain available for future testing.)

Tried as instructed and the first time I tried the game it worked just fine. I tried it again and the "Upgrade Flash" message appeared again. I can't understand it. Now I have this test2024 there and again it still didn't work. Don't know what to do now, go back to the other browser I can use with the game. I have tried, Vivaldi, Duckduckgo browser and Microsoft Edge and the game works fine on all three. I changed nothing on any of them. But I do not get the message on those and never have.

Thank you for testing. It's puzzling that the game's behavior changed between the first run and the second run. I may need to try it myself if there isn't any explanation from the game publisher.

I don't understand it either. I would like to know the difference between Firefox and all the others. There has to be something.

Just tried a new profile once again and again got the Upgrade Flash. I think now the last time I tried I just didn't give it long enough to appear. When it happens it doesn't happen immediately but maybe 2 to 3 seconds once you get to the first page. Obviously it IS Firefox and the page and not my profile. Alas, I shall give up my long time favorite browser for good. Thanks for your attempt to help.